1. Field of the Invention
The present invention relates to a process for producing a rectangular strip from a hot rolled, round rod by passing the rod consecutively through a series of rolling mills. More specifically, the invention relates to a process which employs inter-cooling between consecutive rolling mills as well as strip tension control to produce a strip which exceeds currently achievable maximum width to thickness ratios.
2. Description of the Related Art
Finned tubes are employed in many commercial applications as a means to allow heat exchange to occur between mediums of different temperatures without necessitating commingling of the mediums. One common design for finned tubes employs a fin constructed of a continuous strip of metal fin material. In cross section, this strip is rectangular in dimension, having a width exceeding its thickness. To form the finned tube, the strip is helically wound around the exterior surface of a hollow tube and secured thereto so that one of the shorter sides of the cross-sectional rectangular shaped strip is secured adjacent to the tube's exterior surface and the width of the strip extends outward away from and perpendicular to the tube's exterior surface.
For optimum performance of the finned tube, it is desirable to employ strips having a width to thickness ratio (calculated by dividing the width by the thickness) of approximately 17 to 1. For example, one common strip size has a width of 0.80 inches and a thickness of 0.047 inches resulting in a width to thickness ratio of approximately 17.2 to 1.
The current method for producing strips of the desired dimensions is to begin with a sheet of continuous cold rolled metal, often carbon steel, of the desired thickness. The sheet is then slit into strips of the desired width. During manufacture of the cold rolled sheets, the sheets are uniformly rolled to close tolerances. The price of these sheets reflects the time and effort required to roll them to close tolerances. When the cost of slitting the sheets is added to the cost of the sheets, the total cost to produce strips according to current methods is quite expensive.
The cost for producing strips could be reduced considerably if the strips could be made from less expensive hot rolled rod instead of from the cold rolled sheets. Current methods for converting a hot rolled rod into a strip generally consists of passing a rod consecutively through a series of rolling mills. Each rolling mill further flattens and widens the rod until the desired strip dimension is achieved. However, certain dimensional limitations are encountered when employing currently known methods for converting rods into strips.
Specifically, current methods for converting a rod into a strip are limited to creation of strips with width to thickness ratios of less than approximately 12 to 1. When current methods are employed to create strips having width to thickness ratios exceeding approximately 12 to 1, for example having ratios of 15 to 1 or 17 to 1, during the rolling process the metal strip develops severe edge fractures. These fractures cause the strip to break. The cause these fractures appear to be two-fold. First, as the metal rod is passed through each consecutive rolling mill, heat is added to the metal until the metal finally reaches a temperature at which it fails, causing it to fracture or break. Second, in addition to the heat stress exerted on the metal, mechanical stress is inflicted on the metal by the tug on the rod from the subsequent rolling mill as it pulls the rod toward the mill. This mechanical stress seems to contribute to the metal failure which is observed. Regardless of the cause of the failures, until now the practical upper limit for rolling rod into strips has been the 12 to 1 width to thickness ratio. Because strips having higher width to thickness ratios than 12 to 1 are necessary for construction of finned tubes, production of strips for this use from hot rolled rod has not until now been a viable option.
The present invention addresses this problem of limited width to thickness ratio by providing a method for producing strips from rods which utilizes cooling and feed backward tension controls between adjacent rolling mills in order to produce strips having width to thickness ratios of up to approximately 17 to 1.